Postfix relay to Office 365 external recipients not delivered

I have an issue with Postfix relay to MS Office 365. I can send to internal addresses, but not external. I receive the following error message:

550 5.7.1 Client does not have permissions to send as this sender (in reply to end of DATA command))

I have setup a Microsoft SMTP server and it works fine, both internal and external, but Postfix will not deliver to external addresses. I know it has to be a configuration issue. I just can't pinpoint what it is. I really would prefer to use Postfix. I've used it before and never had any issues, but if I don't figure this out, I will have to go with MS

Here is my main.cf:

# See /usr/share/postfix/main.cf.dist for a commented, more complete version

# Debian specific: Specifying a file name will cause the first
# line of that file to be used as the name. The Debian default
# is /etc/mailname.
myorigin = /etc/mailname

smtpd_banner = $myhostname ESMTP $mail_name (Ubuntu)
biff = no

# appending .domain is the MUA's job.
append_dot_mydomain = no

# Uncomment the next line to generate "delayed mail" warnings
#delay_warning_time = 4h

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